Demyelinating conditions are a group of neurological disorders in which the lipid protein composite substance myelin surrounding axons of neurons which increases their ability to conduct electrical signals degenerates. This group of conditions includes acute and chronic encephalomyelitis, optic neuritis, transverse myelitis, Devic's disease, the leucodystrophies, multiple sclerosis, progressive multifocal leukoencephalopathy, central pontine myelinolysis, neuromyelitits optica, diffuse cerebral sclerosis of Schilder, acute and subacute necrotizing haemorrhagic encephalitis.
The most well-known of these conditions is multiple sclerosis which affects 350,000 Americans and is, with the exception of trauma, the most frequent cause of neurologic disability in early to middle adulthood. There are generally considered to be 3 forms of multiple sclerosis: relapsing, progressive, and inactive. The disease is characterized by selective demyelination of CNS axons, inflammation, and gliosis. The cause of multiple sclerosis is unknown but is thought to have an autoimmune etiology initiated by exposure to a virus in genetically predisposed individuals.
Multiple sclerosis derives its name from the multiple scarred areas visible on macroscopic examination of the brain. Areas of tissue affected in this manner are called plaques which range in size between 1 mm and several centimetres. Demyelinating lesions are historical evidence of the occurrence of or the continued presence of perivascular lesions. Occasionally, plaques are also present in gray matter. The multiple sclerosis lesion is defined as including both perivascular and demyelinating lesions. Demyelination, along with causing conduction abnormalities between neurons, can, in severe cases, lead to premature death.
Myelin is produced by specialized cells (generically referred to as glia). In the CNS, the main myelin-producing glia are named oligodendrocytes. Injury to myelin sheaths surrounding axons may interrupt communication between neurons and produce myelin breakdown. When myelin sheaths or oligodendrocytes sustain injury, entire segments of myelin degenerate, and their remnants are phagocytosed by macrophages, and to a much lesser degree, by astrocytes. This is called “primary demyelination” if most axons remain uninjured and is characteristic of the myelin breakdown seen in multiple sclerosis. In multiple sclerosis, and in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, encephalitogenic leukocytes penetrate the blood brain barrier and damage the myelin sheath of nerve fibres (Raine, 1991, 1994; Martin and McFarland, 1995; Cannella and Raine, 1995; Brosnan and Raine, 1996).
The β7 integrin subfamily of cell adhesion molecules consists of two members, α4β7 (Holzmann and Weissman, 1989; Yuan et al., 1992) and αEβ7 (Parker et al., 1992; Krissansen et al., 1992), whose expression is restricted to leukocytes. α4β7 mediates the adherence of lymphocytes to high endothelial venules (HEV) via its preferred ligand mucosal addressin cell adhesion molecule-1 (MAdCAM-1, Berlin et al., 1993; Briskin et al., 1993; Yang et al., 1995), whereas αEβ7 mediates the adhesion of intraepithelial lymphocytes of the intestine to the intestinal epithelium via an interaction with E-cadherin (Karecla et al., 1995; Cepek et al., 1994, Berg et al., 1999).
In gene knockout mice deficient in the β7 integrin subfamily, the gut-associated lymphoid tissue (GALT) is severely impaired due to a failure of β7−/−lymphocytes to adhere to blood vessel walls at the site of transmigration into the GALT (Wagner et al., 1996). Further, in non-obese diabetic (NOD) mice, treatment with anti-β7 or anti-MAdCAM-1 mAbs provides protection against the spontaneous development of diabetes and insulitis, presumably by blocking lymphocyte migration into the inflamed pancreas (Yang et al., 1997). These examples demonstrate that α4β7-MAdCAM-1 interactions play critical roles in allowing the entry of leukocytes into chronically inflamed tissues.
A role for α4 integrins in the establishment of EAE was demonstrated by in vivo administration of anti-α4 integrin antibodies which diminished the paralysis associated with EAE (Yednock et al., 1992; Baron et al., 1993). Further, the suppressive effects of TNFbp on the development of EAE appears to correlate with down-regulation of VCAM-1/α4β1 (Selmaj et al., 1998). A humanized anti-integrin α4 subunit antibody prevented the development of new lesions in a recent clinical trial (Tubridy et al., 1999). Hence α4-integrins play a crucial role in mediating the pathogenesis of demyelinating disease of the CNS.
An antibody directed against the integrin β7 subunit, the partner chain for the α4 subunit, greatly attenuates a non-remitting form of EAE, induced by adoptive transfer of myelin oliogodendrocyte peptide (MOG35-55)-stimulated T cells (Kanwar et al., 2000a). Combinational treatment with both anti-β7 and α4 integrin subunit antibodies lead to more rapid and complete remission than that obtained with anti-α4 antibody alone, potentially implicating a role for αEβ7 in disease progression. Remission correlated with the down-regulation of the vascular addressins VCAM-1, MAdCAM-1, and ICAM-1 on cerebral blood vessels. Attenuated forms of disease were induced by adoptive transfer of either wild-type encephalitogenic T cells to β7-deficient gene knockout mice, or of β7−/−encephalitogenic T cells to wild-type recipients. The former indicates that β7+ve recruited cells contribute to disease progression. Thus α4β1, α4β7, and αEβ7 integrins may all play a contributory role in the progression of chronic forms of demyelinating disease, and together with their ligands could be potential targets for improved treatment of some forms of multiple sclerosis.
MAdCAM-1, the preferential ligand for α4β7, is predominantly expressed on specialized HEV formed at chronically inflamed sites (Streeter et al., 1988), and is particularly noticeable on HEV-like endothelia in relapsing brain lesions of mice with EAE (Cannella et al., 1991). An antibody directed against MAdCAM-1 prevented the development of a progressive, non-remitting form of EAE, actively induced by injection of myelin oligodendrocyte peptide (MOG35-55) autoantigen (Kanwar et al., 2000b). Combinational treatment with both anti-MAdCAM-1, VCAM-1, and ICAM-1 (ligand for integrin LFA-1) mAbs led to more rapid remission than that obtained with anti-MAdCAM-1 antibody alone. However, neither MAdCAM-1 monotherapy, nor combinational antibody blockade was preventative when administered late in the course of disease progression. Nevertheless, MAdCAM-1 plays a major contributory role in the progression of chronic EAE, and is a potential therapeutic target for the treatment of MS. Anti-vascular addressin therapy must be given early in the course of disease prior to the establishment of irreversible damage if it is to be effective as a single treatment modality.
Integrin-mediated entry of leukocytes into the CNS leads to markedly elevated concentrations of glutamate, a major excitatory amino acid neurotransmitter, in the cerebrospinal fluid of patients suffering from multiple sclerosis (Stover et al., 1997). Glutamate receptors of the AMPA/kainate class are expressed on oligodendrocytes. Low concentrations of glutamate, AMPA, or kainate kill oligodendrocytes in vitro, indicating that glutamate injures oligodendrocytes thereby reducing myelination during EAE (McDonald et al., 1998).
The AMPA/kainate receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX) inhibits the excitotoxic effects of glutamate on oligodendrocytes, and slightly ameliorates the pathogenesis of EAE (Smith et al., 2000; Pitt et al., 2000).
GPE is a tripeptide consisting of amino acids Gly-Pro-Glu. It and its di-peptide derivatives Gly-Pro and Pro-Glu were first disclosed by Sara et al in EP 0366638. The suggestion made by Sara et al is that GPE has neuromodulatory properties (able to affect the electrical properties of neurons).
GPE has also been established as having neuroprotective properties and therefore has utility in the prevention or inhibition of neuronal and glial cell death (WO 95/17204, AU 700838).
GPE has also been established as being able to upregulate neural enzymes and therefore has utility in increasing the effective amount of choline acetyl transferase (ChAT), nitric oxide synthetase (NOS), glutamic acid decarboxylase (GAD) (WO 98/14202) and tyrosine hydroxylase (WO 99/65509) in the brain.
GPE has also been established as having cross-receptor activity in that it inhibits glutamate binding to the N-methyl-D-aspartate (NMDA) receptor (U.S. Pat. No. 5,804,550) and prevents neuronal death in the hippocampus injured by NMDA (Saura et al., 1999).
Current drug management of multiple sclerosis consists of immunosuppression to arrest the disease process and for arresting the symptoms. No drugs are currently successful in intervening in the disease process.
One approach that has been tried has been to administer neuroprotective agents to repair the damage to the CNS, such as the kainate antagonist NBQX as described in Smith et al., 2000 and Pitt et al., 2000. This research demonstrated that administration of NBQX at the early stages of disease in an animal model of the multiple sclerosis could attenuate but not block the disease process.
It is an object of the present invention to provide a method of treating demyelinating diseases which goes some way towards overcoming the disadvantages of the prior art, and in particular which is useful in treating such diseases when in an advanced state, or at least to provide the public with a useful choice.
It is a further object of the invention to provide a method for treating or preventing plaque formation and the demyelination of axons so that the affected areas of the CNS are protected from damage resulting from demyelinating disease including multiple sclerosis.